1. Field of Invention
This invention relates to an azimuth adaptive phased array sonar for use in a medical or underwater ultrasonic imaging system, and more particularly to an azimuth adaptive phased array sonar capable of suppressing grating lobe ghosts or artifacts.
2. Description of Prior Art
Known in the art are phased array sonars having an array probe comprising an array of a plurality of transducer elements (hereinafter called "elements") energized with an appropriate phase arrangement for transmitting ultrasonic waves into, or receiving ultrasonic waves from, an objective space while controlling the beam direction of the waves.
The prior phased array sonars, however, are plagued with ghosts (or artifacts) produced by side lobes, particularly grating side lobes.
Ghosts produced by side lobes appear in a direction determined primarily by the ratio of element-to-element pitch to wavelength of the transmitted and received ultrasonic waves, and at a level determined by the total number of activated elements. In principle, these ghosts cannot be reduced by improving the resolution or quantization error of a delay map for phased element energization, or by improving the uniformity of gain and ultrasonic wave transmission and reception efficiencies. Apopization of the transducer elements only results in sharply cut skirts ML.sub.2 (see FIG. 1) while sacrificing the width of a main lobe ML.sub.1. It also fails to be effective for base lines D.sub.o (FIG. 1) or grating side lobes GSL. The base lines D.sub.o are governed by the total number of transducer elements and the uniformity or deviation, in every meaning thereof, from a theoretical or ideal value. Another factor greatly involved in the grating side lobes is waveform (or a frequency spectrum) received for echo examination, as is apparent from the principle of generation of grating side lobes, illustrated in FIG. 2.
Turning to FIG. 2, elements T.sub.1 through T.sub.6 are arrayed at a pitch d and are simultaneously energized to produce a main lobe in front of the array. At the same time, waves are brought to be inphase in directions of .+-..theta. satisfying d.multidot.sin .theta..apprxeq..lambda.(.lambda. is the wavelength), in addition to the front of the array, to produce a strong sensitivity in such directions (thus, generating grating lobes).
Where the transmitted and recived waves have a better coherence, the grating side lobes become larger and sharper. Where complete impulse waves are transmitted and received, no grating lobes are generated, but added waveforms from all related elements are not sufficiently cancelled out on base lines D.sub.o and remain appreciably to be continuous side lobes.
In case the band width of transmitted and received waveforms is too wide or extends into a high-frequency range in a phased array system, grating lobes tend to arise at an angular position smaller than angle .theta. at a central frequency f.sub.o of the ultrasonic waves.
No problem arises when an ultrasonic beam is directed in front of the array, by selecting the azimuth of possible grating lobes, to be slightly outside (for example .+-.50.degree.) of the range of angles of a field of view, or an azimuth range to be detected (for example .+-.45.degree.). However, when an ultrasonic beam is steered and scanned, grating lobes come up in the field of view, to thereby produce ghosts.
One solution to the above problem, would be to control the gain of an amplifier circuit for receiving and amplifying an echo signal. With this solution, however, the signal-to-noise ratio would not be improved, and no increased advantage or sufficient advantage and optimization would be reached, and best visibility would surely not be attained.